Oprah Winfrey’s Book Club lasted for 15 years and endorsed 70 books, which sold millions. But what effect did it have on the broader market? Did it get people reading more, or less? Via Tyler Cowen, economist Craig Garthwaite suggests that Oprah might’ve actually hurt overall book sales.

There’s no question that Oprah’s blessing was good for individual titles. According to Garthwaite’s research, overall sales of an Oprah book shot up 400 percent in the weeks after the endorsement and stayed high for awhile. What’s more, people also rushed out to buy other books by Oprah-endorsed authors. (Garthwaite notes that the effect here was similar to the boost an author gets from a positive review in the New York Times.) But, oddly enough, aggregate sales of all books tended to decline in the period after an Oprah endorsement.